Figure 1 :The effects of stimulus intensity level on ERPs elicited by rapidly-presented stimuli (about every two seconds) during the waking and sleeping states. These data are from the Cz electrode placement. Note that the ERP waveforms during NREM sleep (stages N2 and N3) are traced at half amplitude compared to wakefulness and REM sleep conditions. In the waking state, as stimulus intensity level was lowered, the amplitude of N1 gradually decreased. A small amplitude P3a was elicited but only by the loud 80 dB SPL stimulus. During NREM (stages N2 and N3) sleep, N1 was at baseline level while during REM sleep, a significant, but much reduced N1 was elicited. On the other hand, large amplitude, P2, was apparent during NREM sleep and its amplitude gradually decreased as stimulus intensity was reduced. A large amplitude N350 was also apparent during NREM sleep, a deflection that was much attenuated during REM sleep. N350 rapidly declined in amplitude as the intensity level of the stimulus was reduced.